This invention concerns particulate media used in a bed for the purification of water or other liquids.
It has been known for some time that solids can be removed from a liquid by passing the liquid through a bed of particulate media. Examples of such methods are described in U.S. Pat. Nos. 4,608,181 (Hsiung et al.), 4,725,367 (McKim et al.), 4,780,219 (Witek), and 5,009,776 (Banks).
Heretofore, the media has been selected, for convenience, from among commercially available products intended for other uses. For example, some filters have been made using polypropylene or polyethylene pellets of the type that are sold to supply injection molding machines. While such prior media have worked, they are not optimized for operation in liquid purification.
Alternatively, water purification apparatuses have employed beds of plastic chunks which are made by comminuting larger bodies of a plastic material. For example, media has been made by melting polypropylene beads to form blocks of polypropylene which are subsequently cooled and ground up. The resulting polypropylene chunks had the properties described in U.S. Pat. No. 4,608,181 (Hsiung et al.). While plastic chunks can be manufactured to have excellent properties for liquid purification, they are expensive to make because energy must be consumed to melt the plastic material to form the large bodies. And, as a result of the heating, portions of the material may be altered undesireably in composition or properties. Also, because such chunks are made using a grinding process, plastic fines are produced. These fines must be removed before the media chunks can be used in a liquid treatment vessel. If the media is not cleaned properly, the fines will clog screens and other apparatus thereby causing premature headloss. Cleaning to remove fines is typically conducted in the field, so there is also a problem with environmentally proper disposal of the waste plastic fines.
Thus, there is a need for a particulate media that is produced affordably and yet provides excellent results when used for liquid purification.